Unified channel access for supporting quality of service (QoS) in a local area Network

ABSTRACT

Contention communications often requires a station to wait an inordinate amount of time before the station is able to transmit its data successfully. In many applications, an extended delay is not acceptable. Contention-free communications in a contention period allows a hybrid coordinator (HC) to schedule contention-free access to a communications medium so that extended delays may be eliminated, and to coordinate contention access to the medium so that better throughput and delay performance is achieved. A method for creating contention-free communications within a contention communications period is presented, along with adaptive algorithms for contention access during the same contention period.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to the provisional applicationsentitled “Probability Based Distributed Coordination Function (p-dcf)for Prioritized Quality of Service”, filed Nov. 27, 2000, Serial Number60/253,252 and “A Probability Based Distributed Coordination Function(p-dcf) for Prioritized Quality of Service”, filed Nov. 1, 2000, SerialNumber 60/244,993, which provisional applications are incorporatedherein by reference.

[0002] This invention is related to commonly-assigned patentapplications: “Shared Communications Channel Access In an OverlappingCoverage Environment”, filed Sep. 28, 2001, Ser. No. ______, attorneydocket number TI-32700, and “Adaptive Algorithms for Optimal Control ofContention Access”, filed Sep. 28, 2001, Ser. No. ______, attorneydocket number TI-32377. These applications are incorporated herein byreference in their entirety.

FIELD OF THE INVENTION

[0003] This invention relates generally to networks and moreparticularly to providing hybrid contention-free and contentioncommunications in a communications network that shares access of acommon communications medium among multiple user stations.

BACKGROUND OF THE INVENTION

[0004] Communications networks use a transmission medium to transmitinformation in the form of computer data, voice, music, video, etc.,from one station to another. The communications medium may be a wiredlink, a fiber optic link, or a wireless link. The wireless link mayinclude, but is not limited to, radio frequency, infrared, laser light,and microwave. The network may, in fact, use a combination of differentcommunications links.

[0005] With the exception of a small number of networks that usededicated communications links between each station, most informationnetworks use a shared transmission medium to carry the transmittedinformation. Examples of information networks using a sharedtransmission medium include: Ethernet, token ring, and wireless Ethernet(IEEE 802.11).

[0006] However, by sharing a communications medium between multiplestations, there are situations that arise when stations are required towait a significant amount of time before they are able to transmit theirdata. Additionally, situations exist when simultaneous transmissionsfrom different stations occur and result in the mutual destruction ofthe transmissions. Such situations are undesirable in providing qualityof service (QoS) to multimedia data transfers and in making efficientuse of scarce spectrum on a wireless medium.

[0007] For some applications, such as voice telephony, videotele-conferencing, and other real-time, bi-directional, interactiveapplications, extended transfer times can severely and rapidly degradethe performance of the applications to a level that is unacceptable. Forexample, in voice telephony applications, if the delay between one userspeaking and another user listening is greater than a few milliseconds,the delay becomes noticeable to the users and the users' satisfactionlevel for the telephone connection begins to drop.

[0008] One way to ensure that applications requiring a low maximumnetwork latency receive the level of service that they require is toimplement some form of QoS transfers of data traffic between stations.In many networks with QoS transfers, communications traffic in a networkare partitioned into multiple categories and the categories areparameterized or prioritized according to their specific performancerequirements. For example, traffic carrying a telephone conversationbetween two users will be given a higher priority than traffic carryingdata for a file transfer between two computers. By creating categoriesfor the traffic, parameterizing and prioritizing the differentcategories and ensuring that traffic of higher QoS demands or higherpriority receives better service, these networks offer and meetperformance guarantees.

[0009] In a network that uses a shared communications medium, onecommonly used technique to ensure a minimum network performance level isto have a centralized controller controlling access to thecommunications medium instead of simply relying on a distributedalgorithm or random chance to provide access control. At an interval ofadjustable duration, the centralized controller polls the stations withdata to transmit and grants each one of them a specified duration on thecommunications medium during which they are free to transmit their datawithout fear of collisions. Each polled station is then guaranteed tohave time on the medium in line with its QoS expectations. This methodis sometimes referred to as contention-free communications.

[0010] The opposite of contention-free communications is communicationswith contention, or contention communications. During contentioncommunications, each station with data to transmit must contend withother stations for access to the communications medium. Algorithms, fromsimple to complex, arbitrate access to the communications medium.However, since the algorithms are non-deterministic and are usuallybased on a first-come-first-served paradigm, the wait that the stationsmust endure cannot be predicted, nor is the rate at which the stationscan transmit their data. Therefore, communications by distributedcontention cannot be used to implement QoS transfers. This is due to thefact that contention communications generally results in communicationswith low throughput and large delay and jitter.

[0011] In the IEEE 802.11 wireless local area network (LAN) technicalspecifications, provisions for both contention-free and contentioncommunications have been provided in two separate communicationsperiods. Each beacon interval is partitioned into a contention-freeperiod (CFP) and a contention period (CP) for contention-free andcontention communications, respectively, with frame exchanges betweenstations in the two periods employing different access rules and frameformats. As a result, QoS traffic transfer is complex to implement,channel throughput efficiency is relatively low, and co-channelinterference mitigation and bandwidth sharing are not straightforward.Furthermore, in IEEE 802.11, the CFP is an option and mostimplementations of IEEE 802.11 wireless LANs do not supportcontention-free communications.

[0012] A need has therefore arisen for a methodology for providinghybrid contention-free and contention communications during a CP over ashared communications medium on a demand driven basis.

SUMMARY OF THE INVENTION

[0013] In one aspect, the present invention provides a method forproviding contention-free transmissions during a contention period in ashared communications medium comprising the steps of capturing theshared communications medium, then permitting a recipient station anopportunity to transmit, and once the recipient station completes itstransmission, recapturing the shared communications medium.

[0014] A preferred embodiment of the present invention has a principaladvantage in that it permits and enables contention-free communicationsto take place in a contention period that does not normally supportcontention-free communications. By having contention-freecommunications, the network is able to implement a QoS plan and supporta wide variety of communications applications that require guaranteedlow-latency and certain data rate communications.

[0015] A preferred embodiment of the present invention has an additionaladvantage in that it still allows for contention communications tooccur, in the midst of contention-free communications.

[0016] A preferred embodiment of the present invention has yet anotheradvantage in that it provides an adaptive contention communicationsparadigm that has been shown to be equivalent to exponential backoff infairness, but makes better use of available network bandwidth anddegrades more gracefully when the number of stations in the networkincreases.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above features of the present invention will be more clearlyunderstood from consideration of the following descriptions inconnection with accompanying drawings in which:

[0018]FIG. 1 is a diagram displaying a typical (prior art) configurationof a wireless local area network;

[0019]FIG. 2 is a diagram displaying the basic structure (prior art) ofa beacon interval according to the IEEE 802.11 technical specifications,version 1999;

[0020]FIG. 3 is a diagram displaying the use of contention-freecommunications in a contention period according to a preferredembodiment of the present invention;

[0021]FIG. 4 is a diagram displaying contention-free communicationsusing tokens during a contention period according to a preferredembodiment of the present invention;

[0022]FIG. 5 is a diagram displaying an adaptive, probability-basedcontention access algorithm according to a preferred embodiment of thepresent invention;

[0023]FIG. 6 is a diagram illustrating an adaptive, backoff-basedcontention access algorithm according to a preferred embodiment of thepresent invention;

[0024]FIG. 7a is a block diagram illustrating a hybrid coordinatoraccording to a preferred embodiment of the present invention; and

[0025]FIG. 7b is a block diagram illustrating a processor internal tothe hybrid coordinator displayed in FIG. 7a according to a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0026] The making and use of the various embodiments are discussed belowin detail. However, it should be appreciated that the present inventionprovides many applicable inventive concepts which can be embodied in awide variety of specific contexts. The specific embodiments discussedare is merely illustrative of specific ways to make and use theinvention, and do not limit the scope of the invention.

[0027] Sharing a communications medium is a necessity for a majority ofthe communications networks (networks) available today. Only in a smallnumber of networks are there enough resources to permit dedicatingcommunications media between pairs of users. For most purposes,dedicating a connection between pairs of users is an inefficient use ofbandwidth resources. Sharing a communications medium between multipleusers allow for more efficient use of the medium, because when one usermay be idle, another user may have data to transmit. Sharing is alsomore cost efficient because a smaller amount of the media is needed tosupport the information network. Note that this is also true forwireless, over-the-air networks, where if sharing were not used, thenmore “air”, i.e., spectrum, must be dedicated to supporting the network.

[0028] However, sharing a communications medium between multiplestations means that in certain circumstances, more than one stationcould desire access to the medium at the same time or a station maydesire access when the medium is already busy. This is known ascontention and contention leads to collision and waiting. Because onlyone station should have access to the communications medium at any giventime, the other stations with data to transmit should wait until thefirst station is either finished transmitting or its allotted time isexpired. However, since the stations do not have other channels overwhich they can coordinate their transmission times other than thechannel they use for their data transmissions, the stations may transmitat the same time resulting in no data being successfully received. Suchcollisions waste channel bandwidth and further delay the traffictransfer.

[0029] Long delays leads to communications with large latencies. Thereare many communications applications that cannot tolerate high networklatency. Examples include voice telephony applications, videotele-conferencing, and other real-time, bi-directional and interactiveapplications. These applications require a network that provides a lowmaximum network latency, a minimal data transfer rate, and other QoSexpectations.

[0030] One step devised to meet QoS expectations involves categorizingtraffic in the network and assigning QoS parameters and/or priorities tothe different traffic categories. Network traffic with the low networklatency requirements are assigned higher priorities to ensure that theyare serviced before traffic with less stringent network latencyrequirements, hence the higher priority traffic is required to waitless. Traffic with lower priorities are sometimes required to wait anextended amount of time, but only applications that are not sensitive toextended wait-times are assigned low priorities. Applications such asdata and file transfers are assigned low priorities, while applicationssuch as voice and video transmissions are assigned high priorities.

[0031] Another step involves the use of contention-free communications.This is similar to the communications method used in token ringnetworks, where a station can communicate only when it has the “token.”In networks implementing contention-free communications, a centralizedcontroller (sometimes referred to as a point coordinator or a hybridcoordinator) communicates to the stations in the network, one at a time,telling the station that it may communicate for a specified amount oftime. The station then is free to transmit for the specified duration.The station can transmit to any other station in the network withoutfear of a collision.

[0032] Referring now to FIG. 1, a diagram (prior art) of a typicalwireless local area network (LAN) installation according to the IEEE802.11 technical standard, “ANSI/IEEE Std 802.11, 1999 Edition;Information technology—Telecommunications and information exchangebetween systems—Local and metropolitan area networks—Specificrequirements. Part 11: Wireless LAN Medium Access Control (MAC) andPhysical Layer (PHY) Specifications,” which is incorporated herein byreference. FIG. 1 provides an illustration of the basic building blocksof an IEEE 802.11 network.

[0033]FIG. 1 displays a first basic service set (BSS) 110 and a secondBSS 120. A BSS is the basic building block of an IEEE 802.11 network andcan be thought of as a coverage area within which member stations mayparticipate in direct communications. A BSS is started, formed, andmaintained by an access point (AP). BSS 110 corresponds to AP 130 andBSS 120 corresponds to AP 140. An AP is a station that is connected to adistribution system (DS) 150. A DS allows multiple BSSs to interconnectwith one another and form an extended service set (ESS) 160. The mediumused in a DS may be the same as the medium used in the BSSs or it may bedifferent, e.g., the medium used in the BSSs may be wireless radiofrequency (RF) while a DS may use fiber optic. Internal to BSS 110 is anAP 130 and a wireless station (STA) 170 while internal to BSS 120 is anAP 140 and a STA 180. A BSS may contain more than two stations (e.g., amaximum of about 20 stations per BSS is typical today), but it will haveone AP.

[0034] As shown in FIG. 1, BSS 110 is connected to DS 150 via the accesspoint 130 and the second access point 140 connects BSS 120 to DS 150. Itshould be noted that an access point also contains a wireless stationand can be addressed like any other wireless station.

[0035] Stations within a BSS, for example, stations 130 and 170 in BSS110, may communicate with one another without interfering with stationsin other BSSs. However, the stations within a BSS cannot simplycommunicate whenever they wish; they must follow an established set ofrules designed to minimize collisions and maximize performance.

[0036] A user may be thought of as a device or an entity that uses astation to communicate with other users who are using other stations tocommunicate. Therefore, in the remainder of this discussion, the termsstations and users will be used interchangeably without loss ofinformation.

[0037] In an IEEE 802.11 wireless LAN, data, management, and controltraffic are transmitted in what are called “units.” Data and controltraffic transmitted between two stations are called Medium AccessControl (MAC) protocol data units (MPDU), while management traffictransmitted between two stations are called MAC management protocol dataunits (MMPDU). A unit may be fragmented if it is too large to fit withina single MAC frame and therefore may be fragmented into multiple MACframes.

[0038] Timing is a crucial aspect of an IEEE 802.11 wireless LAN. Spansof time are used to prevent or allow certain types of communications.Other spans of time are used to begin and end communications. The mostcommonly referred time spans are SIFS, PIFS, and DIFS. A SIFS is a shortinter-frame space and is typically the smallest time span in the LAN. APIFS is a point coordinating function (PCF) inter-frame space. A PIFS isequal to one SIFS plus one slot time. A DIFS is a distributedcoordinating function (DCF) inter-frame space. A DIFS is equal to onePIFS plus one slot time. A slot time is the maximum amount of time for astation to sense a frame, due to signal processing delay and propagationdelay, that is transmitted from any other station within a BSS.

[0039] As described previously, a station may transmit in one of twoways. The station may transmit by contention-free communications startedand controlled by a hybrid coordinator (HC). The HC may be a componentof an access point or it may be a separate entity on the network.However, the HC should be part of the same BSS as the stations that itis controlling. Transmissions during contention-free communications areensured to be free of collisions because only one station within a givenBSS has access to the communications medium at a given time. Duringcontention-free communications either the station containing the HC orthe station polled by the HC can transmit at a given time. Once astation has been polled, it is given access to the medium for aspecified amount of time and is free to transmit information to anydestination for the specified duration.

[0040] Alternatively, the station may transmit by contentioncommunications coordinated by the HC. In order to transmit by contentioncommunications, the station must first determine if the medium is idleand its backoff timer must be zero. If either condition is not met, thenthe station cannot transmit. However, even if both conditions are met,collisions may still occur, since more than one station may haveattempted to transmit at the same time. Transmissions by contentioncommunications typically are afflicted with collisions that will requireone or more retransmissions after an extended delay.

[0041] The present invention, as described below, is presented as anextension of an existing distributed coordination function (DCF) in theIEEE 802.11 technical standard. Therefore, for discussion purposes,terminology used in the discussion will reflect terminology used in theIEEE 802.11 technical standard. However, the ideas embodied in thepresent invention have application in other information networks wherethere is a desire to implement contention-free communications in acontention period. Therefore, the present invention should not beconstrued as being applicable only to a wireless network that adheres tothe IEEE 802.11 technical standard.

[0042] According to the IEEE 802.11 technical standard, bothcontention-free and contention communications are supported.Contention-free communications is supported during a contention-freeperiod (CFP) while contention communications is supported during acontention period (CP). Unfortunately for implementing QoS transfers,the CFP is an option in the IEEE 802.11 technical standard. Even if theCFP is provided, it uses a different set of access rules and frameformats than the CP. As a result, there are many IEEE 802.11 compliantwireless local area networks (LANS) that do not offer a contention-freeperiod. In such networks, network latencies are generally large andspectrum utilization efficiencies are generally poor due to contentionand collision.

[0043] Referring now to FIG. 2, a diagram illustrates a basic framestructure for an IEEE 802.11 wireless LAN. The diagram displays a singlebeacon interval 200 as a function of time. A beacon interval 200 in anIEEE 802.11 wireless LAN begins with a target beacon transmission time(TBTT) that represents a desired (targeted) time for the appearance of abeacon frame 210. The beacon frame 210 denotes the actual beginning of abeacon interval 200 in an IEEE 802.11 wireless LAN.

[0044] According to the IEEE 802.11 technical standard, the beaconinterval 200 can begin with a CFP that lasts for up to a specifiedmaximum duration. However, the CFP may also be of duration zero (0).This means that the CFP may be non-existent. If the CFP is non-zero induration, then the end of the CFP is marked by a CF-End frame 220.

[0045] The contention-free period, according to the IEEE 802.11technical standards, allows for transmissions to occur within a BSSwithout the possibility of collisions from other transmissions fromwithin the BSS. This is enabled by using a central controller, referredto as a point coordinator, that controls all transmissions in the BSS.The point coordinator is part of an access point in the BSS. The pointcoordinator may transmit a data frame from itself, or transmit a pollingframe, or a polling frame with data, to a station in the BSS. The polledstation transmits a MAC frame one SIFS time period after receiving thepolling frame. If the station has more than one MAC frame fortransmission, the station may send another request to the pointcoordinator to request additional time. The request may be piggybackedinto the frame sent in response to the polling frame or it may betransmitted separately at a later time. After one station completes itstransmission, the point coordinator allows itself or another stationaccess to the communications medium.

[0046] The CF-End frame 220 marks the end of the CFP and the beginningof the CP. The duration of the CP is equal to the overall actual beaconinterval duration minus the duration of the CFP. Therefore, the CP maylast the entire beacon interval 200 (if there is no CFP).

[0047] The CP begins with the end of the CFP or the beacon frame 210 ifthere is no CFP. According to the IEEE 802.11 technical standard, astation in the BSS may begin to contend for access to the communicationsmedium not earlier than one DIFS period after the communications mediumbecomes idle. In general, transmissions during the CP are achieved bywaiting for the medium to become idle for at least one DIFS period.

[0048] In IEEE 802.1 1, the network designers are required to implementthe contention access method but not necessarily the contention-freeaccess method, since the former is specified to be mandatory and thelatter is optional. According to the mandatory method, when a stationhas information to transmit, it checks the medium (for IEEE 802.11 inthe United States, the medium spectrum are in the 2.4 and 5.7 GHzindustrial-scientific-medical (ISM) frequency bands) to see if themedium is idle. The designers have defined two different states of idle.The first state of idle is known as “physical idle” and it is when thereare no actual transmissions being performed on the medium. The secondstate of idle is known as “virtual idle” and it is when there are noanticipated transmissions from users on the medium. The medium busy timedue to anticipated transmissions is encoded in a “duration” field ofeach transmitted frame, and is updated at listening stations with a“network allocation vector” (NAV). Both states of idle must be met forthe medium to be considered idle.

[0049] Prior to a new transmission, the user selects a random backofftime from an interval [0, CW), which includes 0 but excludes CW, whereCW=CWmin+1 for a new transmission and CWmin is a predetermined valuethat is specified in the technical specifications. The interval iscommonly referred to as a contention window. This randomly selectedbackoff time is placed into a backoff timer that begins decrementingafter the medium is determined to have been idle for a DIFS interval,where DIFS is also a predetermined value defined in the standard.However, the backoff timer only decrements when the medium remains idle(both states of idle). If the medium is no longer idle, then the backofftimer stops decrementing; the backoff timer resumes decrementing afterthe medium has become idle for a DIFS.

[0050] Once the backoff timer reaches zero, the user transmits a frame.If a collision occurs, the user selects a new random backoff time from acontention window that is twice as large as the previous contentionwindow, [0, 2*CW). After the backoff timer expires with the new backofftime, the user transmits the same frame again. If the transmission failsyet again, the user selects another random backoff time from acontention window that is twice as large as the previous contentionwindow, [0, 4*CW). When the contention window reaches a maximum size,[0, CWmax], where CWmax is yet another predetermined value specified inIEEE 802.11, the contention window will no longer increase in size,regardless of the occurrence of any future collisions of the frame. Thisbackoff process is known as truncated binary exponential backoff andcontinues until the transmission is successful. The frame may be droppedif it reaches a retry limit or a maximum lifetime, which are values alsodefined by the IEEE 802.11 standard. After the frame is successfullytransmitted or after the frame is dropped, the user will perform a newbackoff chosen from the initial contention window [0, CW), withCW=CWmin+1, in preparation for the transmission of another new frame.

[0051] The end of the contention period is marked by another TBTT, at orafter which a next beacon frame 240 may be transmitted by the pointcoordinator. The transmission from a station or/and its acknowledgmentmay go beyond the next TBTT, thus delaying the arrival of a next beaconframe.

[0052] As discussed previously, in the CP, a station with data totransmit may be required to wait a long period of time prior to beingable to transmit. Additionally, the contention access algorithm asspecified by the IEEE 802.11 technical standards is based purely onbackoff. Therefore, there is no consideration given for how long amessage has been waiting for transmission. Because of these factors andothers, the CP as currently specified in the technical standards is notsuitable for supporting traffic expecting performance guarantees.

[0053] Referring now to FIG. 3, a time-space diagram illustratescontention-free communications implemented in a CP according to apreferred embodiment of the present invention. FIG. 3 illustrates thestatus of a single beacon interval 300 as a function of time, with frametraffic originating from an HC displayed above a horizontal line 305 andframe traffic originating from stations displayed below the horizontalline 305.

[0054] As discussed previously, the beacon interval 300 begins with abeacon 310 that also denotes the beginning of an optionalcontention-free period. If the communications frame has acontention-free period, then the end of the contention-free period ismarked by an end of CFP frame 320. If there were no CFPs or if the CFPhas ended, then a CP begins.

[0055] According to the IEEE 802.11 technical standard, once the CPbegins, stations with traffic to transmit may begin contending foraccess to the communications medium one DIFS period after the mediumbecomes idle.

[0056] However, if the communications medium becomes busy again withinone DIFS period, then none of the stations will be allowed to contendfor access.

[0057] According to a preferred embodiment of the present invention, anHC, also sometimes interchangeably referred to as a centralizedcontroller, an access point (AP), or an enhanced access point (EAP), maytake control of the communications medium one PIFS period after themedium becomes idle. Therefore, if the HC transmits a MAC frame one PIFSperiod after the medium becomes idle, the HC obtains control of themedium, since the other stations will sense the medium to be busy oneslot time later, that is, those stations will find the medium to be busyagain within one DIFS period of the last transmission on the medium andthus will not transmit. Therefore, the HC essentially startscontention-free communications in the CP. Within the contention-freecommunications all frames are separated by one SIFS period, furtherpreventing contention and collision by non-HC stations from occurring.

[0058] According to a preferred embodiment of the present invention, theHC may pre-empt contention for the communications medium by the stationsin the BSS during the CP by transmitting after the medium is idle for aPIFS period 330 rather than a DIFS period (as is required for contendingstations). With the HC in control of the communications medium, the HCcan create contention-free transmissions for itself and for otherstations in the BSS by polling. In another preferred embodiment of thepresent invention, the HC may start contention-free communications inthe CP and preempt contention from other stations by transmitting afterthe medium has been idle for one SIFS period, following the end of aframe exchange sequence transmitted through contention access bystations within the same BSS.

[0059] After the medium has been idle for a PIFS or SIFS period asdescribed above, the HC may send a broadcast (one source to alldestinations) or multicast (one source to many destinations) QoS Dataframe, a directed (one source to one destination) QoS Data frame, or adirected QoS {Data+}CF-Poll frame. The notation {Data+} means that theframe may optionally contain data, and if it does not, the presentinvention will remain operable. FIG. 3 displays a QoS CF-Poll frame 340being transmitted to station 1 at the end of a PIFS idle period 330.

[0060] Inside the QoS CF-Poll frame 340, the HC specifies an amount oftime, called a transmission window (TW), that has been reserved for thestation (station 1). The TW is a sum of the amount of time allotted tothe station plus the time required to transmit one QoS CF-Ack frame plusone SIFS period. The TW is equivalent to the amount of time that the HChas allotted for the station to transmit its data plus the amount oftime to acknowledge receipt of is the transmission from the polledstation (the time required to transmit one QoS CF-Ack frame) plus theamount of time the acknowledging station (HC or another station) waits(the SIFS period) prior to transmitting the QoS CF-Ack frame. The HCalso specifies in the frame 340 a duration field equal to the value ofTW plus one SIFS (the time between the end of the poll frame and thebeginning of the poll response).

[0061] The polled station transmits a QoS Data frame 345 after one SIFSof receiving the poll frame, and may use all the specified time for itsown transmission if the transmission requires no acknowledgment (i.e.,no QoS CF-Ack frame will be returned). The station sets the durationfield of the frame 345 to the duration value in the polling frame 340minus the sum of one SIFS and the calculated transmission time of theframe 345. If the station sends more than one frame within a given TW,it will set the duration field in each successively transmitted frameequal to the duration value in the last transmitted frame subtracted bythe sum of one SIFS and the transmission time of the frame to be sent.The station receiving a transmission from the polled station sends a QoSCF-Ack frame one SIFS after the end of that transmission ifacknowledgment is required, setting the duration field in the QoS CF-Ackframe to zero or to a nonzero value if the HC is to transmit anotherframe within a SIFS or PIFS interval.

[0062] If the HC desires to retain control of the medium after thepolled station has completed with its transmission and received itsacknowledgment (if acknowledgment is required), the HC can send anotherframe to the same station or another station one SIFS later. The HC mayuse QoS Data+CF-Ack 350 to include data in an acknowledgment it isrequired to send to the previously transmitting station, setting theduration to be the transmission time of a QoS CF-Ack frame plus oneSIFS. The receiving station, if required to acknowledge, will send a QoSCF-Ack frame 355 back to the HC one SIFS later, setting the durationfield equal to zero. The HC may also send a QoS Data+CF-Poll 360 to anystation (station 4), with the station responding one SIFS later with aQoS Data+CF-Ack frame, a reservation request (RR) frame, or a pollrequest PR frame 365, requesting for a larger transmission window.

[0063] Since not every station in the network has data to transmit, itwould be inefficient for the HC to transmit QoS CF-Poll frames to eachstation in the network and allocating unneeded time on thecommunications medium. Instead, the HC maintains a list of stations thathave data to transmit and wish to use contention free communications.Stations with a burst of data to transmit, i.e., at least two MSDUs,desiring to use contention-free communications, must request allocationsof time from the HC. To do so, the station transmits a poll request (PR)to the HC. Poll requests may be transmitted to the HC viacontention-free or contention communications. In each poll request, thestation includes a field that contains an amount of time required totransmit the burst.

[0064] Stations with a single MSDU to transmit have an option of usingcontention-free communications. Although contention-free communicationsis not required for stations with more than one MSDU, the performanceincrease gained by using contention-free communications makes it muchmore desirable to use contention-free communications for cases where astation has more than one MSDU to transmit. The performance increasegained by using contention-free communications for a single MSDU is notas great and therefore, the decision to use contention-freecommunications is left up to the station. Of course, if a station needsto transmit the single MSDU by a certain time, it would be wise to usecontention-free communications, regardless of any performance benefitthat may result.

[0065] In FIG. 3, station 4 sent a PR frame 365 to the HC. Upon receiptof the PR, the HC sends a QoS {Data+}CF-Ack+CF Poll frame 370 to thestation that had sent the PR frame. Included in the QoS{Data+}CF-Ack+CF-Poll frame is a transmission window that is less thanor equal to the time needed to transmit the size of the burst indicatedin the PR frame. Station 4 thus sends a QoS Data frame 375 one SIFSlater. If needed, the HC will send more QoS {Data+}{CF-Ack+}CF-Pollframes to the station at a later time (depending on some scheduling ornetwork load balancing algorithm executing in the HC), until the burstis empty. The HC releases the control of the communications medium atsome point, allowing other stations to contend for access to the mediumone DIFS after the end of the contention-free communications. However,the HC may regain the medium in the remaining CP 380 by transmitting aframe after one PIFS of medium idle interval, thus starting anothermini-period of contention-free communications within the CP.

[0066] The use of poll request and poll frames is an efficient way toimplement communications between the HC and a non-HC station. However,if there are more than two non-HC stations communicating together, thenthe use of poll requests and CF-Poll frames becomes inefficient andunwieldy due to the large number of poll requests and CF-Poll framesbeing transmitted.

[0067] To support this type of communications, the present inventionprovides for the use of tokens in contention-free communications. Tokensare received frames and some special qualifications that are used todetermine if the receiving station has the right to transmit next. Onlythe station with the token may transmit. The token is passed from thefirst station to a second when the first station's transmission iscomplete.

[0068] According to a preferred embodiment of the present invention, theHC may allow a token-based communication structure to take place bytransmitting a poll frame to a station with a More Data bit in the frameset to zero. With the More Data bit set to zero, the HC is not requiringthe station to return a frame back to the HC, which would result in theHC regaining immediate control of the communications medium. Instead,the station is free to send a frame to another station.

[0069] Referring now to FIG. 4, a space-time diagram illustratescontention-free communications during a CP of a shared communicationsmedium. FIG. 4 illustrates the status of a beacon interval as a functionof time. At the beginning of the CP, the HC gains control of thecommunications medium by transmitting a QoS Data+CF-Poll frame 415 tostation 2. In frame 415, the More Data bit is set to one, which willforce station 2 to return control of the medium to the HC by sending aframe back to the HC but not to another station.

[0070] Station 2 responds to the QoS Data+CF-Poll by transmitting itsdata and returning a CF-Ack to the HC in frame QoS Data+CF-Ack 420. TheHC then transmits a QoS Data+CF-Ack+CF-Poll frame 425 to station 1 whereCF-Ack indicates receipt of the previous frame 420. In frame 425, theMore Data bit is set to zero, which allows station 1 to transfer controlof the communications medium to a station other than the HC. Station 1transmits a QoS Data+CF-Ack frame 430 to station 4, where CF-Ackacknowledges receipt of the previous frame 425. Station 4 takes over themedium by transmitting a QoS Data+CF-Ack frame 435 to station 5, withthe CF-Ack acknowledging receipt of the previous frame 430. Station 5takes over now and continues the chain by transmitting a QoS Data+CF-Ackframe 440 back to station 1, also acknowledging station 4 for the frame435. The chain ends when station 1 transmits a QoS CF-Ack frame 445 backto station 5.

[0071] According to a preferred embodiment of the present invention,there is no maximum to the number of stations permitted in chain offrames using tokens to pass control of the communications medium.However, an upper limit on the number of frames that can be transmittedis determined by the duration permitted the originating station by theHC, the duration of the CP, and ultimately the duration of a beaconinterval. Because no data frames can be transmitted past the TBTT, achain of frames using tokens can only be no longer in duration than aremaining CP.

[0072] According to another preferred embodiment of the presentinvention, a token using chain of frames ends when a station that haspreviously received a token receives a second frame in the same chain offrames. This means that a chain of stations using tokens for accesscontrol cannot transmit data frames to a station more than one time in asingle chain.

[0073] The use of tokens is useful in applications such as voicetelephony where a station transmits to another station and expects aresponse from that same station back. The use of tokens in these typesof applications can reduce the control overhead and therefore increasethe overall network throughput.

[0074] Referring back to FIG. 4, after the transmission of the QoSCF-Ack frame 445, the contention-free period ends. After a DIFS periodof being idle, stations with traffic to transmit begin using contentioncommunications. These stations either were not able to transmit a pollrequest to the HC or chose not to do so. Contention communications usingcontention access will be discussed later in the specification.

[0075] When contention communications completed, the HC regains controlof the communications medium by transmitting a QoS Data frame 455 tostation 3 at the end of a PIFS idle period. The transmission from the HCto station 3 begins a second contention-free communications periodwithin the same CP. Station 3 replies to frame 455 with a poll requestframe 460. The poll request frame 460 was transmitted duringcontention-free communications since station 3 had exclusive access tothe communications medium. If station 3 had not been in exclusive accessto the communications medium, station 3 would have had to wait untilcontention communications begins and attempt to transmit a poll requestvia contention communications.

[0076] In response to the poll request frame 460, the HC transmits a QoSCF-Ack+CF-Poll frame 465 to station 3. The CF-Ack is used as anacknowledgment for the poll request frame 460 while the CF-Poll givesstation exclusive access to the communications medium for a specifiedamount of is time. Station 3 transmits its data and acknowledges thereceipt of frame 465 with QoS Data+CF-Ack frame 470. The HC transmitsdata to station 2 and acknowledges the receipt of station 3's QoS Dataframe 470 with a QoS Data+CF-Ack frame 475. A lone acknowledgment frame,QoS CF-Ack frame 480 is transmitted from station 2 to the HC toacknowledge the QoS Data+CF-Ack frame 475.

[0077] The second contention-free communication period ends. Afteranother DIFS period of the communication medium being idle, a secondcontention communications period 485 begins. The second contentioncommunications period 485 ends when it is determined that no datatraffic can be transmitted prior to the arrival of the next TBTT.

[0078] Contention-free communications in the CP is intended primarilyfor the HC to send MPDUs and/or MMPDUs to stations and/or to pollstations for frame transfers from those stations. To a lesser extent,contention-free communications is for a station to use the receipt of aframe as a token to send an MPDU or an MMPDU. Since the primary purposeof contention-free communications is not for general communications inthe network, it is possible that there is insufficient traffic for acontention-free communications period to consume an entire contentionperiod.

[0079]FIG. 4 displays an example of the different communications methodsthat are permitted during the contention period and should be viewed asbeing only representative of what could actually happen during acontention period. However, there are no specific requirements thatspecify how many contention and contention-free periods should occurduring a single contention period. If there were no need forcontention-free communications during a contention period, then therewould not be any contention-free communications during that contentionperiod.

[0080] While contention-free communications is good for ensuringlow-latency communications, a need is still present for contentioncommunications. For short bursts or single frame transmissions,contention communications can be a good, low overhead way to transmitinformation. Additionally, in networks with a low to moderate number ofstations, contention communications provides a simple method to reliablytransmit data. Also, for stations that are not in a list of stationsdesiring contention-free communications, contention communications isrequired so that the station can transmit a poll request (PR) to the HCto be added to the list of stations desiring contention-freecommunications. Therefore, contention communications cannot be entirelyeliminated.

[0081] According to a preferred embodiment of the present invention,contention communications categorizes the traffic in the network intodifferent traffic categories that are then prioritized. The trafficcategories are prioritized depending on desired performance levels foreach category. For example, to provide a traffic category with a lowoverall network latency, a high priority is assigned. From thepriorities, specific permission probabilities may then be assigned. Eachtraffic category receives a permission probability, TCPP_(i), whereTCPP_(i) is the traffic category permission probability (TCPP) for thei-th traffic category. According to a preferred embodiment of thepresent invention, a centralized controller, such as the HC can be usedto assign the TCPPs. According to another preferred embodiment of thepresent invention, the individual stations themselves are able to assignthe TCPPs.

[0082] Regardless of which entity assigns the TCPPs, an important aspectof the present invention is that the changes of TCPPS are not fixed butadaptive. The TCPPs can and should be changed to reflect current networkconditions. This adaptive nature helps the network react to changingnetwork conditions and traffic loads. Updating the TCPPs may beperformed at fixed time intervals or it may be performed when specificnetwork performance metrics fall outside specified ranges. As with theassignment of the TCPPs discussed above, an HC or the individualstations may perform the updates.

[0083] Referring now to FIG. 5, a diagram illustrates an algorithm 500employed by a contention station for contention communications usingTCPPs executing on a station according to a preferred embodiment of thepresent invention. According to a preferred embodiment of the presentinvention, a station with frames of traffic queued in various trafficcategory queues calculates its overall permission probability, PP, bycalculating a sum of the individual TCPPs (510). According to apreferred embodiment of the present invention, the traffic categorieswith empty queues have zeros assigned as their TCPPs.

[0084] The communications medium is then checked to see if it is idle(520) for the requisite period of time (e.g., a DIFS period). If thecommunications medium is busy, the algorithm 500 remains in block 520 towait until the communications medium becomes idle. If the communicationsmedium is idle, then the contending station checks to see if any of theTCPPs have been updated (530). As discussed above, the TCPPs may beupdated by the HC or the individual stations.

[0085] If the TCPPs have been updated, the contending stationrecalculates the overall permission probability (540). The contendingstation then determines if it has permission to transmit a frame (550).To determine if it has permission to transmit, the contending stationselects a random number, X, from the range [0, 1). This random number,X, is then compared against the overall permission probability. PP. Ifthe random number, X, is less than or equal to the overall permissionprobability, PP, then the station has permission to transmit.

[0086] After being permitted to transmit, the contending station mustselect a frame from a traffic category queue and transmits it (560). Theframe is selected from traffic category N, where N meets the criteria:

[0087] if 0<X<TCPP_(l), then N=0; else${{{if}\quad {\sum\limits_{i = 0}^{M - 1}\quad {TCPP}_{i}}} < X \leq {\sum\limits_{i = 0}^{M}\quad {TCPP}_{i}}},{{{then}\quad N} = {M.}}$

[0088] Here, TCPP_(i) is the TCPP for traffic category i and is set tozero if traffic category i has no traffic to send from the station.

[0089] After transmitting the frame, the contending station waits for anacknowledgment (560). If the contending station receives anacknowledgment, the transmission was successful. If the contendingstation does not receive an acknowledgment, then the transmission was afailure and the contending station may attempt to retransmit the frame.According to a preferred embodiment of the present invention, thealgorithm 500 treats the retransmission of the failed frame the same wayas if the retransmission was an initial attempt at transmitting a frame.

[0090] After transmitting the frame and verifying the status of thetransmission, the contending station checks to see if it needs tofurther contend for access to the communications medium (570). If thereis no further need to contend, then the algorithm 500 terminates.

[0091] If, in block 550, the contending station was not permitted totransmit, then it checks to see if the time slot was idle (580). If thetime slot was idle, is then the contending station can try to transmitagain at the next time slot by returning to block 530. If the time slothad been busy, then the contending station must wait for the medium tobecome idle once again (520).

[0092] Referring now to FIG. 6, a diagram illustrates an algorithm 600used by a contending station for contention communications using backoffexecuting on a station according to a preferred embodiment of thepresent invention. A station with frames of traffic queued in varioustraffic category queues calculates its overall permission probability,PP, by calculating a sum of the individual TCPPs and uses the overallpermission probability to calculate a backoff time, J (610). The backofftime, J, is calculated using the expression:

J=└log(X)/log(1−PP)┘

[0093] where └Y┘ denotes the largest integer number not exceeding Y.According to a preferred embodiment of the present invention, thetraffic categories with empty queues have zeros assigned as their TCPPs.

[0094] The communications medium is then checked to see if it is idle(620) for the requisite period of time (e.g., a DIFS period). If thecommunications medium is busy, the algorithm 600 remains in block 620 towait until the communications medium becomes idle. If the communicationsmedium is idle, then the contending station checks to see if any of theTCPPs have been updated (630). As discussed above, the TCPPs may beupdated by the centralized controller or the individual stations.

[0095] If the TCPPs have been updated, the contending stationrecalculates the overall permission probability, PP, and the backofftime, J (640). The contending station then determines if it haspermission to transmit a frame (650). The contending station loads thebackoff time, J, into a backoff timer. The backoff timer decrements thevalue stored in it by one each time an idle slot passes. Therefore, thecontending station is forced to wait J idle time slots before it ispermitted to transmit.

[0096] After being permitted to transmit, the contending station mustselect a frame from a traffic category queue and transmits it (660). Theframe is selected from traffic category N, where N meets the criteria:

[0097] if0<C*X≦TCPP_(l), then N=0; else${{{if}\quad {\sum\limits_{i = 0}^{M - 1}\quad {TCPP}_{i}}} < {C*X} \leq {\sum\limits_{i = 0}^{M}\quad {TCPP}_{i}}},{{{then}\quad N} = {M.}}$

[0098] Here, ${C = {\sum\limits_{i = 0}^{Z}\quad {TCPP}_{i}}},$

[0099] Z is the number of traffic categories, and TCPP_(i) is set tozero for traffic categories having no traffic to send from the station.

[0100] After transmitting the frame, the contending station waits for anacknowledgment (660). If the contending station receives anacknowledgment, the transmission was successful. If the contendingstation does not receive an acknowledgment, then the transmission was afailure and the contending station may attempt to retransmit the frame.According to a preferred embodiment of the present invention, thealgorithm 600 treats the retransmission of the failed frame the same wayas if the retransmission was an initial attempt at transmitting a frame.

[0101] After transmitting the frame and verifying the status of thetransmission, the contending station checks to see if it needs tofurther contend for the communications medium (670). If there is nofurther need to contend, then the algorithm 600 terminates.

[0102] If, in block 650, the contending station was not permitted totransmit, then it checks to see if the time slot was idle (680). If thetime slot was idle, then the backoff timer decrements the value storedin it by one. The contending station can try to transmit again at thenext time slot by returning to block 630. If the time slot had beenbusy, then the contending station must wait for the medium to becomeidle once again (620).

[0103] Referring now to FIG. 7a, a block diagram illustrates an HC 700with hardware support for contention-free communications during acontention period in accordance with a preferred embodiment of thepresent invention. The HC 700 has a processor 710 that is responsiblefor controlling the contention-free communications and for coordinatingcontention communications, which includes but is not limited to:maintaining a list of stations desiring contention-free communications,adding and removing stations from the list, scheduling the order inwhich stations in the list are served; and updating contention accessparameters such as TCPPs or contention windows. The processor 710 iscoupled to a memory unit 715 which can be used to store the list ofstations desiring contention-free communications.

[0104] Also coupled to the processor is a transmit and receive unit 720,commonly referred to as a transceiver. The transmit and receive unit 720is responsible for transmitting and receiving frames. Also coupled tothe processor 710 is a medium sensor unit 725. The medium sensor unit725 is responsible for detecting the state of a communications medium730, either idle or busy. Coupled to the transmit and receive unit 720and the medium sensor 725 is the communications medium 730.

[0105] According to a preferred embodiment of the present invention, themedium sensor unit 725, once it determines the state of thecommunications medium 730, places a value into a register inside theprocessor 710. For example, if the communications medium 730 is busy,then the medium sensor unit 725 will place a certain value into theregister. The medium sensor unit 725 places a different value into theregister if the communications medium 730 is idle. According to anotherpreferred embodiment of the present invention, the medium sensor unit725 asserts a medium status flag one way if the communications medium730 is idle and another way if it is busy.

[0106] According to another preferred embodiment of the presentinvention, the medium sensor unit 725 is internal to the transmit andreceive unit 720. With the medium sensor unit 725 inside the transmitand receive unit 720, the medium sensor unit 725 can make use of some ofthe hardware existing in the transmit and receive unit 720 to determinethe state of the communications medium 730. For example, the mediumsensor unit 725 may use portions of the receiver hardware to detect thestatus of the communications medium 730.

[0107] The communications medium 730 is shown in FIG. 7a as being aphysical, wired connection. However, according to a preferred embodimentof the present invention, the communications medium 730 may be anymedium that is capable of transmitting data. Examples of differentpossible communications medium include, but are not limited to: wire(power line, telephone line, twisted pair, coaxial cable,multi-conductor wire, etc.), fiber optics (single mode and multi-mode),wireless (radio frequency, infrared, microwave, laser light, etc.).

[0108] Referring now to FIG. 7b, a block diagram illustrates theprocessor 710 in greater detail. According to a preferred embodiment ofthe present invention, the processor 710 comprises a list processor 760,a scheduler 765, and a contention coordinator 770. The list processor760, the scheduler 765, and the contention coordinator 770 may befunctions executing inside the processor 710 or they may be dedicatedhardware units inside the processor 710. The list processor 760, thescheduler 765, and the contention coordinator 770 are coupled to anoperational unit 775 of the processor 710, sometimes called a controlunit. The operational unit 775 is responsible for executing programs andperforming functions such as coordinating interactions between variousfunctional units of the processor 710. The operational unit 775 is, inturn, coupled to a bus connecting the processor 710 and its internalcomponents to the remainder of the HC 700.

[0109] The list processor 760 is responsible for maintaining the list ofstations desiring to transmit frames using contention-freecommunications. For example, when the processor 710 requests the listprocessor 760 for a station wanting contention-free communications, thelist processor 760 provides it with the station and relevant informationregarding the station's needs, including how many frames it needs totransmit and at what priorities, etc. When the HC 700 receives a pollrequest from a station, the HC 700 sends the request to the listprocessor 760 who stores the request in the list.

[0110] The scheduler 765 is responsible for ordering the stations andtheir requests for transmission time so that the service provided isfair and no station receives too much or too little of the networkbandwidth. The scheduler 765 operates using a set of prespecified rulesdefining how to order requests of differing priorities and durations.According to a preferred embodiment of the present invention, thescheduler 765 places requests with higher priorities closer to thebeginning of the list and requests for shorter durations are typicallyplaced closer to the beginning of the list than requests for longerdurations.

[0111] The contention coordinator 770 is responsible for updating theTCPPs used by stations for contention communications. Updating the TCPPsmay be performed at fixed time intervals or it may be performed whenspecific network performance metrics fall outside specified ranges. Theupdated values may be broadcast via a beacon frame or another managementframe.

[0112] While this invention has been described with reference toillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications and combinations ofthe illustrative embodiments, as well as other embodiments of theinvention, will be apparent to persons skilled in the art upon referenceto the description. It is therefore intended that the appended claimsencompass any such modifications or embodiments.

What is claimed is:
 1. A method for providing contention freetransmission during a contention period in a shared communicationsmedium comprising: (1) capturing the shared communications medium; (2)permitting a recipient to transmit; and (3) recapturing the sharedcommunications medium after the recipient transmits.
 2. The method ofclaim 1, further comprises the step of (0) waiting for the sharedcommunications medium to become idle prior to capturing the sharedcommunications medium.
 3. The method of claim 1, wherein the capturingstep comprises a hybrid controller transmitting a frame.
 4. The methodof claim 3, wherein the frame is transmitted prior to the expiration ofa point coordination function (PCF) interframe space (PIFS) period afterthe shared communications medium becomes idle.
 5. The method of claim 3,wherein the frame is transmitted prior to the expiration of a shortinterframe space (SIFS) period after the shared communications mediumbecomes idle.
 6. The method of claim 3, wherein the frame is a dataframe.
 7. The method of claim 3, wherein the frame is a control frame.8. The method of claim 3, wherein the frame is a combination data andcontrol frame.
 9. The method of claim 3, further comprising the step of(8) repeating steps (1)-(3) after the shared communications medium hasbeen idle for a PIFS period.
 10. The method of claim 7, wherein theshared communications medium is shared by a plurality of stations, andwherein the control frame contains a duration specifying how long therecipient can transmit, and wherein the recipient may transmit frames toany station as long as the recipient can complete the transmissionwithin the duration specified by the control frame.
 11. The method ofclaim 10, wherein a station receiving a frame from the recipient maytransmit frames to any station as long as the station can complete thetransmission within the duration specified by the control frame.
 12. Themethod of claim 11, wherein if a station receiving a frame haspreviously received a frame within the duration specified by the controlframe, then the data transmission terminates, even if sufficient timeremains in the duration specified by the control frame to transmitadditional frames.
 13. The method of claim 1, wherein the permittingstep comprises: transmitting a frame by the recipient; and transmittinga frame by a hybrid controller.
 14. The method of claim 13, whereinthere are multiple traffic categories, and wherein the recipient maytransmit a frame from any traffic category as long as the recipient cantransmit the frame within a duration specified by a control frame. 15.The method of claim 13, wherein there are multiple traffic categories,and wherein the recipient may transmit multiple data frames of trafficfrom any traffic category as long as the recipient can transmit theframe within a duration specified by a control frame.
 16. The method ofclaim 13, wherein the hybrid controller may begin transmitting a frameone SIFS period after the recipient finishes transmitting.
 17. Themethod of claim 1, wherein the shared communications medium is shared bya plurality of recipients, and the method further comprises the step ofrepeating steps (2)-(3) until each recipient has transmitted all of itsframes.
 18. The method of claim 1, further comprising the step ofrepeating steps (2)-(3) until the contention period ends.
 19. The methodof claim 1, wherein the shared communications medium is shared by aplurality of recipients, and further comprising the step of (4)transmitting control frames to a second recipient after a firstrecipient has finished transmitting, even if the first recipient hasadditional frames to transmit.
 20. The method of claim 19, furthercomprising the step of (5) repeating steps (2)-(4) until each recipienthas transmitted all of its frames.
 21. The method of claim 1, wherein astation becomes a recipient by sending a control frame to a hybridcontroller.
 22. The method of claim 21, wherein upon receipt of thecontrol frame from the station, the hybrid controller places the stationinto a list of recipients.
 23. The method of claim 1, further comprisesthe step of (6) releasing the communications medium after thecontention-free transmission ends.
 24. The method of claim 1, furthercomprises the step of (7) transmitting is frames using contention accessafter the contention-free transmission ends.
 25. The method of claim 24,wherein a hybrid controller coordinates the contention access byupdating and broadcasting contention access parameters for use bycontenting stations in contention communications.
 26. The method ofclaim 25, wherein the updating and broadcasting are performed at fixedtime intervals.
 27. The method of claim 25, wherein the updating andbroadcasting are performed when specific network performance metricsfall outside specified ranges.
 28. A centralized controller comprising:a memory; a processor coupled to the memory, the processor containingcircuitry to manage contention-free access to a communications medium,the processor further comprises: a list processor to maintain a list ofstations desiring contention-free communications; a scheduler to arrangethe serving order of the stations in the list of stations desiringcontention-free communications; a contention coordinator to updatecontention access parameters for use by contending stations incontention communications; the centralized controller further comprises:a transmit/receive unit coupled to the processor, the transmit/receiveunit to transmit and receive data frames from the communications medium;and a medium sensor unit coupled to the processor, the medium sensor todetect a state of the communications medium.
 29. The centralizedcontroller of claim 28, wherein the medium sensor unit is internal tothe transmit/receive unit.
 29. The centralized controller of claim 28,wherein the medium sensor outputs the state of the communications mediumto a memory location in the centralized controller.
 30. The centralizedcontroller of claim 28, wherein the medium sensor asserts a mediumstatus flag depending on the state of the communications medium.
 31. Thecentralized controller of claim 28, wherein the scheduler arranges theorder of serving the stations in a first come first served basis. 32.The centralized controller of claim 28, wherein the scheduler arrangesthe order of serving the stations in a list ordered by traffic category.33. The centralized controller of claim 28, wherein the contentioncoordinator updates the values of contention access parameters forcontention communications.
 34. A communications network comprising: acommunications medium; at least one station, coupled to thecommunications medium, the communications station capable ofcommunicating with other stations; a centralized controller, coupled tothe communications medium, the centralized controller for managingcommunications during a contention-free communications period, thecentralized controller further comprises: a memory; a processor coupledto the memory, the processor containing circuitry to managecontention-free access to a communications medium; a transmit/receiveunit coupled to the processor, the transmit/receive unit to transmit andreceive data frames from the communications medium; and a medium sensorunit coupled to the processor, the medium sensor to detect a state ofthe communications medium.
 35. The communications network of claim 34,wherein the processor further comprises: a list processor to maintain alist of stations desiring contention-free communications; a scheduler toarrange the serving order of the stations in the list of stationsdesiring contention-free communications; and a contention coordinator toupdate contention access parameters for use by contending stations incontention communications.
 36. The communications network of claim 34,wherein the communications medium is radio frequency spectrum.
 37. Thecommunications network of claim 34, wherein the centralized controlleris internal to a communications station.